The Sylvester–Gallai Theorem
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The Sylvester–Gallai Theorem
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Re: The Sylvester–Gallai Theorem
#2Futility closet is fantastic!
Re: The Sylvester–Gallai Theorem
#3> Every finite set of points in the Euclidean plane that is not collinear has a line that passes through exactly two of the points.
I can't make out the point here (no pun). Of course a line can pass through any two points. It could pass through three if those points were collinear but the statement says they're not. So what is the new fact?
Re: The Sylvester–Gallai Theorem
#4> Every finite set of points in the Euclidean plane that is not collinear has a line that passes through exactly two of the points. I can't make out the point here (no pun). Of course a line can pass through any two points. It could pass through three if those points were collinear but the statement says they're not. So what is the new fact?
It’s that the line passes through exactly two points, which if you think about it is not exactly obvious.
Re: The Sylvester–Gallai Theorem
#5> Every finite set of points in the Euclidean plane that is not collinear has a line that passes through exactly two of the points. I can't make out the point here (no pun). Of course a line can pass through any two points. It could pass through three if those points were collinear but the statement says they're not. So what is the new fact?
> So what is the new fact?
For all arbitrarily sized (but finite) sets of not collinear points, there's always a line that passes through exactly two points in the set.
Re: The Sylvester–Gallai Theorem
#6> Every finite set of points in the Euclidean plane that is not collinear has a line that passes through exactly two of the points. I can't make out the point here (no pun). Of course a line can pass through any two points. It could pass through three if those points were collinear but the statement says they're not. So what is the new fact?
It can help to think about theorems like this by restating them as a puzzle asking for a counterexample.
Given N points, N > 2, can you arrange them in a Euclidean plane so that (1) they are not all on the same line, and (2) every line that goes through two of the points must also go through at least one more of the points?
The theorem says that you cannot do this.
Re: The Sylvester–Gallai Theorem
#7> Every finite set of points in the Euclidean plane that is not collinear has a line that passes through exactly two of the points. I can't make out the point here (no pun). Of course a line can pass through any two points. It could pass through three if those points were collinear but the statement says they're not. So what is the new fact?
Try to come up with a set non-colinear points where NO line passes through two and ONLY TWO points and you'll see the value of the statement.
You may think "I'm sure I can arrange these points in a way where EVERY line will cross three or more points" but you will fail if you try unless ALL points are colinear.
Re: The Sylvester–Gallai Theorem
#8I might be too stupid to understand why this is interesting and useful. If it helps I am a working physicist, and a lot of pure math is lost on me. I think I followed this, but I don't know why one would care or this would be interesting.
Re: The Sylvester–Gallai Theorem
#9> Every finite set of points in the Euclidean plane that is not collinear has a line that passes through exactly two of the points. I can't make out the point here (no pun). Of course a line can pass through any two points. It could pass through three if those points were collinear but the statement says they're not. So what is the new fact?
Try to come up with a set non-colinear points where NO line passes through two and ONLY TWO points and you'll see the value of the statement. You may think "I'm sure I can arrange these points in a way where EVERY line will cross three or more points" but you will fail if you try unless ALL points are colinear.
This is true for finite sets. For infinite sets, the Sierpinski triangle is a counterexample.
Re: The Sylvester–Gallai Theorem
#10> Every finite set of points in the Euclidean plane that is not collinear has a line that passes through exactly two of the points. I can't make out the point here (no pun). Of course a line can pass through any two points. It could pass through three if those points were collinear but the statement says they're not. So what is the new fact?
I think what's going on here is that you've misunderstood the theorem's hypothesis. The hypothesis isn't that no three of the points are collinear; rather, it's the weaker statement that there isn't any one single line that all the points lie on. It's true that with your version of the hypothesis the theorem would be trivial; but with the actual hypothesis it is is nontrivial.